It is well known to employ material cutting tools for turning operations wherein chips are removed from workpieces being machined. A turning operation is a machining process for forming external surfaces on a workpiece by the action of a cutting tool, usually on a rotating workpiece. Typically, the workpiece is mounted on a lathe. Most metal removal in lathe turning is accomplished by cutting tools with a single point in contact with the workpiece. These cutting tools may be produced in one piece from a solid bar of tool steel with the appropriate cutting edge ground on one end. They also may be constructed in two pieces, typically including a holder and a turning insert made of carbide or some other hard material. The turning insert of the two-piece tool may be held in place mechanically holder. Brazed, soldered, or welded inserts typically may be resharpened, while inserts held in place mechanically usually are removed, discarded, and replaced with a new, sharp insert after they become worn. Carbide turning inserts manufactured from powdered metals have replaced one-piece ground carbide tools in most turning applications due to their low cost and high wear resistance.
FIG. 1 is a end view of a typical turning operation of a cylindrical workpiece 10 being turned with a two piece cutting tool including a typical turning insert 12 secured in a holder 14. The workpiece 10 is being rotated on a lathe (not shown) about an axis of rotation 16. The turning operation is set up to turn to a depth 18 with a clearance angle 15. FIG. 2 is a plan view of turning insert 12. Turning insert 12 is a diamond shape turning insert with an 80° nose corner angle 26 on nose corner 24. The main cutting edge 22 move in the direction of feed to perform most of the cutting of the workpiece while the secondary cutting edge 20 performs significantly less cutting of the workpiece.
The American National Standards Institute (ANSI) has developed standard nomenclature for identification of replaceable turning inserts. Each ANSI standard insert is identified by a nine digit alphanumeric string that specifically identifies the features of the insert. Insert holders are also designed according to ANSI standards for uniformity. Each holder is designed to accommodate and securely hold ANSI standard inserts. The nine digit alphanumeric string specifies the following features of a turning insert: shape, clearance, tolerance, type, size, thickness, corner, edge condition, and hand. A typical turning insert numerical signature could be, for example, SNMG432AR, and the meaning of an ANSI alphanumeric string for a turning insert will be apparent to those of ordinary skill in the art. ANSI standard turning inserts typically have a nose corner radius of between 1/16 inch and ¼ inch.
The efficiency and quality of turning operations depends on the cutting parameters set on the lathe, the characteristics of the cutting insert, and the characteristics of the material being turned. The relevant cutting parameters include, for example, the feed rate, the lead angle of the turning insert, and workpiece rotational speed. Machinists attempt to optimize these parameters based on the turning insert employed and the material being turned to obtain the highest feed rate while achieving the required surface quality on the finished product.
Tremendous forces are exerted on a turning insert during a typical cutting operation. As the insert begins to cut, it is subjected to large compressive forces. The insert experiences widely varying axial and radial forces as the turning insert moves through the workpiece due to constantly changing chip thicknesses. High axial forces can cause vibration and chatter. Conversely, high radial forces can cause the workpiece to move in the lathe causing poor tolerances and poor surface quality.
The lead angle of the insert on the lathe primarily dictates the relative magnitude of axial and radial cutting forces produced. The lead angle also has a significant influence on the way the radial and axial forces are exerted on the workpiece and the turning insert during a typical machining operation. The lead angle is defined as the angle between the main cutting edge and the direction of feed. The forces that the workpiece exert upon the turning insert change as the lead angle is adjusted. As the lead angle increases, the radial forces decrease and the axial forces increase. Using a lead angle of 45° and a standard radius nose corner insert, the radial and axial forces exerted upon the turning insert are approximately equal. Machinists must attempt to balance these forces in order to optimize surface smoothness and dimensional accuracy.
The shape and features of an insert are also critical to the efficiency and quality of the turning process. The nose radius is a curve defined by the edge of the insert that connects the main cutting edge to the secondary cutting edge. A conventional turning insert has a single radius, with the edge of the insert that connects the main cutting edge and the secondary cutting edge being a segment of a circle with a constant radius. A turning insert with a relatively large radius will produce a finished workpiece with the best surface quality. However, a larger nose radius will increase the cutting forces, both axially and radially, required to perform the turning operation and often results in poor chip control.
Vibration of the workpiece and the lathe may also occur when using a turning insert with a large nose radius in a turning operation. Vibrations may adversely affect the smoothness of the turned metal surface and, also, the service life of the turning insert. Therefore, the use of a turning insert having a large nose radius in metal turning operations is very limited. However, for rough machining operations, an insert with a large nose radius provides the strongest cutting surface and, therefore, the longest service life. Accordingly, round, square or rectangular inserts with a large radius are, typically, chosen for roughing applications.
For metal finishing operations, a triangular, trigon, or diamond-shaped turning insert with a small nose radius is usually selected to produce the smoothest finished surface. Turning inserts with a smaller cutting tip angle (i.e., the angle between the main cutting edge and the secondary cutting edge.) and a smaller nose corner radius allow greater control of the forces generated in the turning operation and provide a smoother surface on the finished workpiece. Such inserts are not as strong as turning inserts with larger radius nose corners and larger cutting tip angles and, therefore, have a higher wear rate and shorter service life.
Accordingly, there exists a need for a turning insert that combines the advantages of both a small nose radius and a larger nose radius. In an attempt to address that need, wiper turning inserts have been developed. As used herein, a “wiper turning insert” is a turning insert that has a radius that is not a curve of constant radius between the main cutting edge and the secondary cutting edge. U.S. Pat. No. 5,634,745, for example, describes a turning insert that includes a nose corner defining at least five circle segments between the main cutting edge and the secondary cutting edge. The design solution provided by the insert of the '745 patent is limited in that the transition between one circle segment and the adjacent segment is abrupt, though tangent, and constrained to certain limitations. The nose corner of the turning insert of the '745 patent is limited in that it may only be described by circle segments. In addition, the largest radius among the multiple circle segments as described in the patent '745 is limited to less than 8 or 10 mm. The research under the present invention indicates that an arc radius beyond 10 mm would not only benefit the surface finish but also reduce the sensitivity of surface finish variations due to inevitable manufacturing inaccuracy of both cutting inserts and tool holders.
Thus, there remains in the art a need for a turning insert that can provide smooth surface finish on a machined surface over a wide range of cutting conditions. There also exists a need for a turning insert that combines the advantages of small nose radius turning insert with the advantages of larger nose radius turning inserts. There is also a need for a method of designing a wiper insert manufacturing a nose corner having smooth no-abrupt transition point sand large radius arcs.